Certain applications including laser projection systems, ray steering applications, optical multiplexers and the like, make use of a narrow collimated laser beam that usually scan across a flat surface along a straight line path. A typical optical scanning system adapted for such purpose employs a tilting flat mirror which deflects the beam. The tilting micro-mirror serves as a central element in many Micro Electro Mechanical Systems (“MEMS”) devices and/or Micro Opto Electro Mechanical Systems (“MOEMS”) devices. For the convenience of the reader, the term “MEMS” as will be referred to hereinafter throughout the specification and claims should be understood to encompass the terms “MEMS” and/or “MOEMS”.
Many of these MEMS devices comprise two types of electro-statically mirrors:
In-plane mirrors—Also known as “Resonance mirrors” are MEMS electrostatic mirrors, usually driven at their respective resonance frequency. The stator and the rotor of these mirrors are composed at the same layer and the mirrors' driving pulses are usually of a rectangular type; and
Staggered mirrors—Also known as “Vertical Mirrors” or “Analog Mirrors” or “DC Mirrors”, are MEMS electrostatic mirrors, which are typically comprised of two different layers, one that comprises the stator while the other comprises the rotor. In some cases, where the stator or the rotor is tilted permanently after manufacturing, only one layer may be used for the stator and the rotor. The staggered mirrors may operate at their resonance frequency or at lower frequencies down to and including DC voltage, and may tilt to any specified angle within a pre-defined range and for any period of time.
Unfortunately the use of the staggered mirrors is still limited due to several difficulties, among which are:                The manufacturing process is more complex than the in-plain mirrors, and requires a high level of precision; and movement is very weak, so the range of the tilt angle is somewhat limited        The electrostatic force which is responsible for the mirror ited.        
In order to increase the staggered mirror tilt angle it is required to decrease the spring torsion constant K which in turn would cause undesirable side effects. Although it is highly recommended that the spring constant K will be very low, it turns out to be a rather complicated task. The following parameters affect spring torsion constant K:
a. The material characteristics—The material in most of the staggered mirrors is pre-defined (usually silicon) and cannot be changed.
b. The spring length—Increasing spring length reduces K, however, that involves increasing silicon area and consequently the overall cost.
c. The spring thickness—Although spring thickness could be different from the comb thickness, it is desirable that the spring thickness is the same as the comb thickness and the layer thickness. This way, fewer processes are required and thickness is accurately known. Unfortunately, in order to facilitate large deflections, the required thickness would be relatively large.
d. The spring width—this is actually the only parameter that practically may be changed (free parameter). Reducing spring width would reduce spring torsion constant K, but also would reduce other characteristic constants like its resistance to lateral forces acting on the mirror, and consequently, lateral movements (and also vertical movements) might seriously affect the performance.
Jer-Liang Andrew Yeh, Hongrui Jiang and Norman C. Tien, in their publication “Integrated Polysilicon and DRIE Bulk Silicon Micromachining for an Electrostatic Torsional Actuator”, J. of Microelectromechanical Systems, Vol. 8, No. 4, December 1999 (incorporated by reference), describe a process for fabricating comb teeth for the moving part (“rotor”) and the non-moving part (“stator”). The rotor teeth lie at different height from the stator teeth. However in this publication, all rotor teeth lie at the same plane while all stator teeth lie at a different plane. Using different planes for the rotor and stator creates torsion forces that tilt the rotor plane.
Dooyoung Hah, Pamela R. Patterson, Hung D. Nguyen, Hiroshi Toshiyoshi and Ming C. Wu describe in their publication “Theory and Experiments of Angular Vertical Comb-Drive Actuators for Scanning Micromirrors”, (IEEE J. of Selected Topics in Quantum Electronics, Vol. 10, No. 3, May/June 2004 p.505 (incorporated by reference)) two types of actuators “AVC”—angular vertical comb drive and “SVC” staggered vertical comb drive. In both cases all rotor teeth lie at the same plane, either parallel to the wafer (SVC) or tilted relative to the wafer (AVC), while all stator teeth lie in a different plane. Using different planes (or angles) for the rotor and stator creates torsion forces that tilt the rotor plane.
Similar structures having two distinct planes, one for the rotor and one for the stator are disclosed in U.S. Pat. No. 7,079,299 (incorporated by reference) and in “A FLAT HIGH-FREQUENCY SCANNING MICROMIRROR” by Robert A. Conant, Jocelyn T. Nee, Kam Y. Lau, and Richard S. Muller from Berkeley Sensor & Actuator Center, University of California, Berkeley, Berkeley, Calif. 94720-1774(incorporated by reference). As mentioned above, one of the major drawbacks with these structures concerns the use of different planes for the rotor and stator, which in turn leads to the development of undesired vertical and transversal forces in addition to torsion forces that tilt the rotor plane.
U.S. Pat. No. 7,089,666 (incorporated by reference) provides tilting mechanism for the rotor by heating springs to plasticity and then, cooling them down to their new quiescent position.
U.S. Pat. No. 7,808,150 (incorporated by reference) discloses tilting mechanism to the stator comb teeth, thus achieving similar effects.
U.S. Pat. No. 7,817,331 and US published application 2008/0316577 (both incorporated by reference) propose to tilt either the moving comb, or the stationary comb.
U.S. Pat. No. 7,538,927 (incorporated by reference) describes a method for fabricating MEMS mirror with two different layers and possibly two different etching techniques to optimize speed. Each layer teeth belongs either to the stationary comb (stator), or to the rotational comb.
U.S. Pat. No. 7,573,022 (incorporated by reference) describes a method for fabricating two vertically offset interdigitated-comb actuato—a fixed comb and a moving comb. Each comb resides at a different layer.
US published application 2003/073261 (incorporated by reference) describes a stationary comb drive and a movable comb drive. Each of the comb drives resides at a different vertical height.
Unfortunately, none of the above publications provides an efficient way to use vertical MEMS mirrors, in a way that would sufficiently overcome the additional electrostatic forces that cause lateral and vertical forces, which in return affect the performance of the staggered mirror.